This invention relates in general to the calibration of data signal channels. More particularly, this invention relates to the automatic calibration of the record/reproduce channel of a magnetic tape recorder. Several circuit components of a reproduce amplifier/equalization circuit of the channel are adjusted simultaneously to effect rapid calibration of the channel.
Signals which are transmitted over a data signal channel, such as the record/reproduce channel of a magnetic tape recorder, are subject to nonlinear frequency response and nonlinear phase response, with the result that such signals are distorted and attenuated. For example, a record/reproduce channel of a magnetic tape recorder includes a reproduce magnetic head, the frequency and phase response of which is affected by many factors. These factors include the length of the reproduce head gap, tape-to-head aerodynamics, tape surface finish and frequency losses in the head's magnetic materials. Thus, it has been the practice to use an amplitude and phase equalization reproduce circuit, to compensate for nonlinear frequency and phase distortion in order to provide a uniform response over the frequency band of interest.
With even the most sophisticated manufacturing processes and controls, head-to-head variations are such, as to require, equalization circuits having adjustable components to optimize the performance of each record/reproduce channel. Thus, as a magnetic head is used, tape running over the head causes wear which changes some of the dimensions effecting magnetic losses and tape-to-head aerodynamics. This in turn changes the head response necessitating periodic adjustments to the equalization circuit. Where these adjustments are performed manually, the adjustment procedure requires several iterations, is time consuming, and requires considerable skill and experience on the part of an operator. These problems are compounded in the case of multispeed playback systems, since the equalization function has to be changed and readjusted for each playback speed. Where the magnetic tape recorder has several channels, manual calibration of each record/reproduce channel for each magnetic tape speed is time-consuming and inefficient.
It has been proposed to effect automatic calibration of the record/reproduce channels of a magnetic tape recorder by using as a calibration signal a psuedorandom digital pattern which contains frequency components necessary for adjusting the reproduce equalizer circuit. In the proposed technique, repetitive adjustments are made at each frequency of interest. Once the calibration signal is within the amplitude error limits at one frequency, a second set of iterative adjustments are made at the next frequency. Since the reproduce equalizer adjustments are interactive, i.e. one adjustment can effect other adjustments, after the amplitude of the second frequency signal is brought within error limits, the routine must branch back to repeat adjustments for the first frequency amplitude. Thereafter, adjustments are made for a third frequency of interest and for phase. In each case the routine must branch back and repeat iterative adjustments for earlier frequency amplitudes brought within error limits. This technique is disadvantageous because of the time required in performing the sequential and multiple iterative adjustments for each channel at each speed.